The fuel cells are devices wherein the reaction energy released by the combination of a fuel (for example, methanol, ethanol, hydrogen or mixtures of the same) with a comburent (for example pure oxygen, air, chlorine or bromine) is not completely converted to thermal energy but also to electric energy in the form of continuous current. In these devices, the fuel is fed to the anode, which takes a negative polarity, and the comburent is fed to the cathode, which conversely takes a positive polarity. The generation of electric energy in the fuel cell is extremely interesting for the high efficiency of fuel utilization as this process is not negatively affected by the limitations of the Carnot cycle, as well as for the minimum environmental impact as concerns dangerous emissions and noise. In the case pure hydrogen is used as the fuel, the environmental impact is close to zero.
Fuel cells may be schematically classified under different categories, characterized by the type of electrolyte separating the anode and cathode compartments and consequently by the operating temperature range. This classification directly reflects on the type of use proposed or expected for these systems. In particular, fuel cells operating at high temperature, that is above 200xc2x0 C., are becoming an alternative source of electric energy in large size power stations, also due to the interesting possibilities of co-generation allowed by the high thermal level. On the other hand, the most interesting type of fuel cell in the field of small range power generation, both for stationary and mobile applications (for example transportation) relies on electrolytes consisting of polymeric ion exchange membranes, the use of which at temperatures above 100xc2x0 C. is conventionally prevented by the need to keep a high internal hydration. Their high current and power density as well as the rapidity in starting-up and achieving full load operation, make the membrane fuel cell much more competitive than any other solution for said applications. However, the limitation of the thermal level permitted by the ion exchange membranes is a severe shortcoming to their commercialization, mainly due to the lack of flexibility of the fuel which can be directly fed to the membrane fuel cell at temperatures below 100xc2x0 C. In fact, the availability of pure hydrogen is limited to a niche of applications wherein this fuel is present as a by-product, as in the case of chlor-alkali electrolysis plants. The present and future market scenario, at least in the medium time, for a large scale commercialization of said system, foresees the use of fuels available at an existing distribution network and easily transportable, such as liquid fuels. The most promising alternatives to pure hydrogen comprise the light alcohols (methanol, ethanol), or the hydrogen-containing gas mixtures coming from steam reforming or partial oxidation of readily available materials, such as natural gas, liquid hydrocarbons (gasoline, gas oil etc.) and the same light alcohols in the case where the direct feed to the fuel cell is not sufficiently effective. However, none of these alternative fuels is reasonably compatible with the present technology of the ion exchange membrane fuel cells operating at low temperature. The light alcohols are negatively affected by insurmountable problems. In fact, on the one hand, their oxidation kinetics at low temperature is dramatically slow, so that the conventional combustion processes have a better efficiency, even if the electrodes of the fuel cell comprise sophisticated catalysts based on noble metals; on the other hand, the high permeability of commercially available membranes causes a loss of alcohol toward the cathode with the overall result of a decrease of the cell voltage with the consequent further loss of electric efficiency. In the case of hydrogen coming from processing of natural gas, alcohols or fossil fuels, a well known problem is the unavoidable presence of carbon monoxide, which, when higher than 10 ppm, can remarkably penalize the performance of the anodic catalysts at the present operating temperatures. Both the problems connected with the oxidation kinetics of alcohols on noble metals, and the poisoning of the latter by traces of carbon monoxide, may be virtually eliminated increasing the operating temperature above 130xc2x0 C. Commercial membranes suitable for industrial applications consist of perfluorocarbosulphonic acids (e.g. Nafion(copyright) membranes commercialized by Du Pont, U.S.A), which cannot be operated at medium temperature for two different reasons; a first temperature threshold, 100xc2x0 C., cannot be trespassed due to the difficulty in maintaining the water management of the system, especially when operating with gaseous reactants at low pressure (a condition imposed by most practical applications to maximize the overall efficiency of the system). A second thermal limit, around 130xc2x0 C., corresponds to the beginning of the transition phase phenomena for most of the perfluorocarbosulphonic acids; above this temperature, crystalline phases begin to segregate inside the polymer giving rise to non homogeneity that affects the electric conductivity and mechanical stability. The nonhomogeneous current distribution, under these conditions, causes thermal hot spots which irreversibly damage the polymer.
An improvement of the water management of perfluorosulfonic membranes was proposed in U.S. Pat. No. 5,523,181. In the case of the fuel cells operating with hydrogen and oxygen or hydrogen and air, the dispersion of fine silica particles inside the polymers favours their water retentive properties, permitting operation of the fuel cells with a reduced water content in the gaseous reactants. In the most favourable cases, the hydration of the membrane may be maintained even without external humidification of the reactant gases, only by the water generated by the reaction. Said invention, at although permitting operation above 100xc2x0 C. in most industrial process conditions (e.g. feeding reactants under low pressure) does not address the problem of the transition phase of the polymers forming the ion exchange membranes, but it greatly simplifies the system around fuel cells operating at low temperatures.
The above considerations apply also to electrochemical cells incorporating an electrode of the type used in fuel cells, in particular the anode fed with hydrogen, hydrogen-containing gases and alcohols. This type of advanced electrochemical cells permits to obtain remarkable reductions in the energy consumption in various fields of application, such as, for example, salt splitting and electrometallurgy.
The present invention is directed to provide polymeric membrane fuel cells suitable for operation at medium temperature (100-160xc2x0 C.), with a large flexibility for the choice of the fuel to be fed to the anode (light alcohols, hydrogen from steam reforming or partial oxidation of alcohols or fossil hydrocarbon fuels, or natural gas).
The use of conventional ion exchange polymers at the above mentioned conditions is made possible by improving the invention described in U.S. Pat. No. 5,523,181, referred to by the technical literature as Stonehart membrane (for example, Fuel Cell Seminar 1996 Proceedings., from page 591), which may be obtained by dispersion of SiO2 (0.01-50% by weight) in a liquid phase suspension of the ion exchange polymer with subsequent redeposition of the obtained mixture to form a film having a controlled thickness (typically from 3 to 200 micrometers depending on the applications).
The improvement provided by the present invention consists in a thermal treatment at a temperature above 130xc2x0 C., and preferably above 150xc2x0 C., for a period of time ranging from 1 to 60 minutes, preferably from 3 to 15 minutes, to obtain a controlled transition phase of part of the polymer; the degree of conversion from the amorphous phase to the crystalline phase may be controlled by X-ray diffraction during the thermal treatment.
It was surprisingly found that, after this treatment, the mechanical properties of the polymer film are remarkably improved and the operation of the fuel cell at temperatures in the range from 100 to 160xc2x0 C. is made possible for prolonged periods without any risk of dehydration or non-controlled transition phases.
In particular, the operation of fuel cells at temperature above 130xc2x0 C. permits a great flexibility in terms of the fuel to be fed to the anode, eliminating the occurrence of poisoning of the noble metal based catalysts by carbon monoxide and other organic compounds, as well as the remarkable increase of the oxidation kinetics of fuels such as methanol and ethanol.
Further, it was surprisingly found that, in the case of cells fed with light alcohols, migration of these fuels through the membrane is remarkably reduced, as indirectly demonstrated by the open circuit voltage which reaches surprisingly high values. The diffusion coefficient of the methanol species was experimentally determined under real working conditions: the obtained value of 1.33.10xe2x88x928 cm2 sxe2x88x921 is about one thousand times lower than the conventional values reported by the scientific literature (e.g. M. W. Verbrugge, J. Electrochem. Soc. 136. No2, 1989, pages 417-423).
It is a main object of the present invention to provide for an improved proton exchange membrane, suitable for operating at temperatures above 100xc2x0 C. with a reduced permeation by alcohols, consisting of a perfluorocarbosulfonic acid having embedded therein silica particles with a concentration by weight comprised between 0.01 and 50%, characterized in that said membrane comprises both an amorphous and a crystalline phase and the ratio thereof is adjusted by means of a controlled thermal treatment at a temperature higher than the glass transition temperature.
It is a further object of the present invention to provide for a membrane electrochemical fuel cell wherein a fuel selected in the group comprising methanol, ethanol, hydrogen coming from the processing of hydrocarbons or alcohols is fed to the anode compartment, said fuel cell comprising the improved membrane of the present invention.
The following detailed description is intended to illustrate the various possible uses of the present invention resorting to some representative examples; however it is to be understood that these examples are not to be considered as a limitation of the same.